Why not just make signatures plain text, no more colors and no more BBcode. This may give those trying to get their first coins a chance to participate in a text-only advertising campaign, which isn't going to pay as much and isn't going to be as obnoxious. You could still have a 2BTC cost to be allowed BBcode, which I assume would result in entries into higher-paying campaigns? Just a thought that I haven't really had time to process, so feel free to rip it apart 
I'm struggling to fully support the idea just because I am against the thought of buying any rank on a forum. The 'donator' status had a purpose at one point in time, but at this point I do not agree with the idea of buying a status that subsequently provides a false sense of trust to others.
Regardless, I'm not sure how else you would stop the activity without having a price set that makes it a much longer ROI and possibly not worth their time. But maybe just get rid of the 'rank' part... or give them the rank, but do not give them 5 coins under their name, access to donator forum, etc. the way the donator status does now. They are not really donating anything, they are trying to participate in paid signature campaigns for a profit... Or just make the rank name "Profiteering Glutton"
Also, I assume this may have been looked at before but maybe ranking in general could be based on the word count or character count of posts as opposed to the quantity of them? Maybe too resource intensive?
I'm struggling to fully support the idea just because I am against the thought of buying any rank on a forum. The 'donator' status had a purpose at one point in time, but at this point I do not agree with the idea of buying a status that subsequently provides a false sense of trust to others.
Regardless, I'm not sure how else you would stop the activity without having a price set that makes it a much longer ROI and possibly not worth their time. But maybe just get rid of the 'rank' part... or give them the rank, but do not give them 5 coins under their name, access to donator forum, etc. the way the donator status does now. They are not really donating anything, they are trying to participate in paid signature campaigns for a profit... Or just make the rank name "Profiteering Glutton"
Also, I assume this may have been looked at before but maybe ranking in general could be based on the word count or character count of posts as opposed to the quantity of them? Maybe too resource intensive?